410 THE MAINTENANCE OF SPECIES 



Lobsters, crabs, spiders, and some insects have tiie uncanny ability 

 of breaking off an injured appendage near its base, a phenomenon 

 known as autotomy. In such instances new appendages are usually 

 regenerated and the animal emerges as a successful contestant in 

 another skirmish in the struggle for existence. Vertebrates, how- 

 ever, show but slight ability to replace lost parts. Of course a 

 break in the skin is soon healed by regeneration, although more 

 extensive damage to the part results merely in the elaboration of 

 some connective tissue and skin and not in complete restoration. 

 A crushed toe, for example, usually necessitates an amputation, for 

 in such cases one never finds a new toe replacing the old. 



It is a rather striking fact that the more limited type of regeneration 

 common among the higher vertebrates is almost indistinguishable 

 from the normal metabolic processes so characteristic of growth and 

 repair. It is only a step from such methods of growth to the highly 

 specialized type known as reproduction. 



Asexual Types of Reproduction 



Budding and fission, or simple cell division, comprise the usual 

 asexual methods of reproduction. A brief consideration of these 

 methods at this point will serve to link regenerative processes with 

 those of higher types of reproduction. The former may be thought of 

 as reproduction by an unequal cell division, a mode of division not 

 infrequently found among one-celled organisms. In more complex 

 organisms, as Hydra, repeated divisions of totipotent cells may occur 

 to produce a bud. Fission merely involves the division of an organ- 

 ism into two or more, usually approximately equal parts. 



Budding 



Organisms which undergo budding might easily be confused with 

 those exhibiting regeneration. These phenomena closely resemble 

 each other, the chief difference being that budding, unlike regener- 

 ation, does not typically result from injury. It is, moreover, an 

 important type of reproduction occurring quite generally in plants 

 as well as widely throughout the lower animal kingdom. 



The fresh-water sponge reproduces by means of two kinds of buds, 

 the first type being liberated to take up a separate existence while 

 the second remains as a kind of internal bud, called a gemmule. It 



